This invention in one of its aspects relates to radiation detectors of the type utilizing photoconductive cells. In another of its aspects the invention pertains to detectors having their photomultiplier tubes mounted within cryogenic coolers.
Photoconductive cells are devices used for detecting or measuring electromagnetic radiation due to variations in the conductivity of certain materials, called photoconductors, by virtue of absorption of radiation by these materials. Such photoconductive cells are used in electronic equipment for many purposes, especially for the detection of electromagnetic radiation having wavelengths greater than those detectable by the human eye, such as infrared radiation. It is known, that resolution and sensitivity of photoconducting light sensitive devices vary inversely with temperature. Hence deleterious effects are obtained if they are operated at elevated temperatures. These effects are mitigated if the radiation sensitive elements in these devices are held at extremely low temperatures. To this end it is desirable to provide cooling means for photoconductive radiation detectors. Photoconductors are generally cooled with liquid nitrogen at -196.degree. C., with liquid hydrogen at -250.degree. C., or with liquid helium at -269.degree. C. to increase their sensitivities. But this is not easy where the detectors are subject to vibrations, such as aircraft instruments.
One type of low temperature photoconductor is fabricated by placing the photomultiplier tube in double-walled vacuum Dewar flask which is then filled with a refrigerant such as dry ice or liquid nitrogen. This device is not completely satisfactory because it restricts the use of the multiplier phototube to one position. To overcome this restriction the detector array has been mounted near a projection or cold finger emanating from a refrigerator, the refrigerator being either an open cycle or closed cycle cooling system. The cooling function of the refrigerator is directed toward the end of a cold finger that extends from the cooler.
More specifically, in a cold finger cooled infared detector, a cold finger extends into a well formed by an inner Dewar wall and contacts the substantially flat portion of the Dewar wall at the bottom of the well adjacent the detector. A disadvantage of this cold finger system is that length of the cold finger is not always the same as the depth of the Dewar well. Therefore, it has been necessary to extend the cold finger by some means to bring it against the inner Dewar wall for efficient cooling.
One previously used means for extending the cold finger involves a spring housing fitting the end of the cold finger with a coupling member physically contacting the inner Dewar wall. A spring within the coupling member urges it against the wall as shown in U.S. Pat. No. 3,851,173. A flexible heat transfer strip within the coupling increases heat transfer between the coupling member and the cold finger to cool the Dewar wall.
The spring-type detector, while providing for conductive cooling, is nevertheless subject to certain disadvantages. As pointed out in U.S. Pat. No. 3,807,188 when a cold finger of a refrigerator is mechanically coupled to a device to be cooled and the device is mounted in or on a glass Dewar, misalignment can cause the Dewar to break. To overcome this problem in U.S. Pat. No. 3,807,188 a conductive neck is used surrounded by a bellows. The neck, in good conductance with the Dewar base plate, is fabricated in a metal such as copper, and it extends into the well toward the cold finger. The bellows, shaped to surround this conductive neck, is directly secured to the cold finger so that misalignment between the conductive neck and the cold finger is compensated for by the flexure of the bellows. Mercury fills the bellows. Once frozen, there is a solid metal-to-metal conductive path from the end of the cold finger, through the conductive neck, to the surface to be cooled.
The two types of Dewar detectors having cold fingers and including flexible construction such as the bellows described in U.S. Pat. No. 3,807,188, and the spring means described in U.S. Pat. No. 3,851,173, do not adequately solve the cryogenic cooling problem because they are in physical contact with the Dewar wall. Mechanical vibrations frequently occur in coolers, and these vibrations can be so severe that even if they do not break the Dewar, the presence of the vibrations can be detected with relatively unsophisticated sound detection equipment, even at fairly large distances. The problem can be greatly overcome by the use of balanced designs and more efficient cooling cycles, but a small component of vibration remains which is transmitted to the Dewar/detector. Aside from the stress problems presented, this residual vibration, termed detector microphonism, affects the quality of the image produced by the radiation detector.
To overcome the vibration problem a recent solution is the development of a cold finger thermal coupling which involves the use of a rigid coupling member contoured to substantially match but not quite touch the inner contour of the well formed by the inner Dewar wall. However, fabrication is critical, partly due to the non-forgiving design and configuration of the assembly. Moreover great care must be exercised when the cryogenic cold finger is inserted in the Dewar well in order to avoid its breakage. This invention relates to still a different solution to the problem, providing a thermal coupler not as subject as prior devices to breakage of the Dewar detector well.